1. Field of the Invention
The present invention relates to a mask inspection apparatus for EUVL (Extremely Ultraviolet Lithography), and in particular, to a mask inspection apparatus and a mask inspection method called an Actinic inspection that uses EUV light with a wavelength of 13.5 nm, which is the same as an exposure wavelength, as illumination light.
2. Description of Related Art
In the lithography technology for miniaturizing semiconductors, currently the ArF lithography that uses an ArF excimer laser with an exposure wavelength of 193 nm as an exposure light source is employed for mass production. A liquid immersion technique in which a gap between an objective lens of an exposure apparatus and a wafer is filled with water in order to improve the resolution, which is called ArF immersion lithography, is becoming widely used for mass production. Development of technology for promoting practical use of EUVL with an exposure wavelength of 13.5 nm in order to further improve miniaturization of the semiconductors has become widespread.
A mask used for EUVL (hereinafter referred to as an EUV mask) has a layered structure in which a multi-layer film for reflecting extreme ultraviolet (extreme ultraviolet used in EUVL will be hereinafter referred to as EUV light) is formed on a substrate made of a low thermal expansion glass. The multi-layer film has a structure in which molybdenum (Mo) and Silicon (Si) are alternately layered in several tens of layers (often referred as a Mo/Si multi-layer film). The multi-layer film can reflect about 65% of EUV light with a wavelength of 13.5 nm, which is vertically incident on the multi-layer.
An absorber that absorbs the EUV light (e.g., tantalum boron nitride (TaBN)) is deposited on the multi-layer film, and then a blank (a mask without a pattern) is formed. Note that a protective film is formed between the absorber and the multi-layer film. After the blank is formed, a resist process is used to form the absorber in a pattern. Thus, a patterned EUV mask is completed.
A blanks inspection apparatus that inspects defects in a blank using an EUV light source is called an ABI (Actinic M blank inspection) apparatus. A basic configuration of an optical system of an ABI apparatus 900 according to related art is shown in FIG. 10.
As shown in FIG. 10, illumination light EUV901 including EUV light with a wavelength of 13.5 nm is emitted from an EUV light source 901 toward a mask M901, which is to be inspected, reflected by large ellipsoidal mirrors 902a and 902b, and then travels while being narrowed like illumination light EUV902. Then, the light is reflected by a drop-in mirror 903a to travel toward the mask M901 side, so that illumination light EUV903 is incident almost vertically on a small area on a surface of a mask blank of the mask M901. Although specularly reflected light reflected by the mask M901 travels in a direction opposite to a direction in which the illumination light EUV903 travels, if there is a defect in the small area, scattered light S901 is generated.
The scattered light S901 travels while spreading at an angle greater than that of the specularly reflected light. Then, the scattered light S901 travels upward around the drop-in mirror 903a, is incident on a concave mirror 905 that constitutes a Schwarzschild optical system 904. The scattered light S901 reflected by the concave mirror 905 is incident on a convex mirror 906 and then reflected by the convex mirror 906. Scattered light S902 reflected by the convex mirror 906 travels upward and is incident on a CCD detector 907.
The Schwarzschild optical system 904 is designed to magnify a small area on the mask blank of the mask M901 that is illuminated by the illumination light EUV903 by about 26 times and project the small area on the CCD detector 907. This enables an observation of a defect present on a surface of the mask blank of the mask M901. The ABI apparatus 900 is described by, for example, Takashi Yamane, et al, “Actinic EUVL M blank inspection and phase defect characterization,” Proceedings of SPIE, Vol. 7379, 73790H (2009).
As described above, in the ABI apparatus 900, light generated from the surface of the mask M901 reaches the CCD detector 907 only when a defect is present. Accordingly, when there is no defect, no signal is output from the CCD detector 907. When there is no defect, an image will be dark. The inspection shown in FIG. 10 is referred to as a dark field inspection.
On the other hand, as shown in FIG. 11, when a defect is detected on the mask M901, an inspection is performed in such a way that specularly reflected light 910b of the illumination light EUV903 that illuminates the small area including a defect is made to be incident on the CCD detector 907 in order to enable a shape and a size of the defect to be examined. Such an inspection is referred to as a bright field inspection. That is, when the illumination light EUV903 is incident on the surface of the mask M901, a drop-in mirror 903b for bright field illumination is adjusted in order for the illumination light EUV903 to illuminate the small area at an oblique angle of incidence of about 6 degrees. Consequently, specularly reflected light 910b is generated.
The specularly reflected light 910b is incident on the Schwarzschild optical system 904 to finally reach the CCD detector 907, thereby enabling an observation of a shape of the defect. However, as defects are small, the shapes thereof cannot be accurately recognized by the magnification of about 26 times, which is made by the Schwarzschild optical system 904. Thus, the small area is further magnified by several tens of times by a planar mirror 908 and a concave mirror 909, and the small area is highly magnified by about several hundreds of times to 1200 times in total. This enables an accurate observation of the shape of the defect. Note that in the ABI apparatus 900, such a function for switching to a magnifying optical system for EUV capable of a high magnification is disclosed in, for example, Japanese Patent No. 5008012.
In most cases, EUV light is absorbed in the atmosphere, and an intensity of the EUV light is reduced. For this reason, exposures and inspections using the EUV light are performed in a vacuum. There is a problem that if the power of the EUV light is large, stains such as carbon compounds (carbon contaminants) or the like are attached to a reflective surface of a mirror that reflects the EUV light, thereby reducing the reflectance. For example, the following cleaning method for removing the stains such as carbon contaminants adhered to the mirror is suggested.
In a method for outputting VUV (Vacuum UV) light from a VUV lamp in an oxygen or ozone atmosphere, firstly, the oxygen or ozone is excited by VUV light, and then atomic oxygen (oxygen radicals) is generated. The generated oxygen radicals decompose stains such as carbon contaminants or the like. Note that the cleaning method for carbon contaminants is disclosed by, for example, Toshihisa Anazawa, et al, “Novel Ozone-based Cleaning Technique for EUV Ms and Optics Carbon Contamination,” International Symposium on Extreme Ultraviolet Lithography (2009) and by Ryo Shibata, et al, “Development of low-pressure UV cleaning technique for carbon contaminated EUV optics,” International Symposium on Extreme Ultraviolet Lithography (2010) etc.
In another method, hydrogen gas is made to flow inside an apparatus, and then hydrogen radicals are generated by EUV light in the exposure light or inspection light. The generated hydrogen radicals decompose stains such as carbon contaminants or the like. An amount of absorption of EUV light by hydrogen is smaller than that by other kinds of gas such as oxygen and the like. Therefore, an advantage of using hydrogen radicals is that mirrors can be cleaned while performing exposure.
One of problems in the ABI apparatus 900 according to the related art will be described based on the ABI apparatus 900 shown in FIGS. 10 and 11. The problem is that stains are easily deposited on reflective surfaces of the drop-in mirrors 903a and 903b. The stains contain, for example, carbon compounds. The stains containing carbon compounds are referred to as carbon contaminants. These stains reduce the reflectance of EUV light and reduce the power of reflected light. It is considered that one of the causes for this is that small quantities of the carbon compounds present inside the ABI apparatus 900, which is under vacuum, are decomposed by the irradiation of the EUV light and then adhered to the reflective surfaces. In particular, the greater the intensity of the EUV light that is incident on the reflective surfaces, the greater the speed at which stains such as carbon contaminants or the like are generated will become.
For example, in the ABI apparatus 900 shown in FIG. 10, a size of a spot (an illuminated spot) of the illumination light EUV 902 that is incident on the drop-in mirror 903a is smaller than those on the ellipsoidal mirrors 902a and 902b by two orders of magnitude by an area ratio. Therefore, an intensity of the illumination light on the illuminated spot will become extremely high. Thus, the generation speed of stains such as carbon contaminants or the like is increased. As described above, there has been a problem in the mask inspection apparatus that stains such as carbon contaminants or the like are generated on the drop-in mirror 903a in a short time.
As shown in FIG. 11, a magnifying optical system with a high magnification is employed in a bright field inspection that is carried out in order to observe a small defect. However, as the intensity of the light reaching the CCD detector 907 is reduced in inverse proportion to a square of the magnification, it is necessary to increase the intensity of illumination light by an order(s) of magnitude so that light with a sufficient intensity will be incident on the CCD detector 907.
It is obvious that although an observation with a high magnification can be carried out with a low intensity of the illumination light, it is necessary to increase the exposure time by the CCD detector 907 to be extremely long. Therefore, inspections are more susceptible to the influence of an environment such as one with vibrations and the like during the long exposure time. It is thus necessary to increase the power of the illumination light in order to observe a defect in a short time. However, when the power of the illumination light is increased, there has been a problem that stains such as carbon contaminants or the like are generated in a shorter time, and thus the reflective surfaces need to be frequently cleaned.
Further, the mirror cleaning method for EUV exposure apparatuses according to the related art is characterized in that mirrors can be cleaned while performing exposure. However, in this method, EUV light with extremely high power is necessary in order to generate hydrogen radicals, and thus it has been difficult to employ this method for an inspection apparatus for masks with a power of EUV light that is low by an order(s) of magnitude.
The present invention has been made to solve such problems, and an object of the present invention is to provide a mask inspection apparatus and a mask inspection method that can prevent a reduction in a reflectance of a drop-in mirror, which is caused by stains such as carbon contaminants or the like, in an Actinic inspection apparatus that uses an EUV light source. Another object of the present invention is to provide a mask inspection apparatus and a mask inspection method that can prevent an interruption of an inspection, which is caused by cleaning of a drop-in mirror.